Anions in trade effluents - Determination of Anions in Natural and Treated Waters - page 597

a solid state membrane electrode. The solid membrane electrode was prepared from
hydrous thorium oxide gel with polystyrene as binder. Although the electrode is non-
Nernstian in nature it can be utilised to estimate sulphate ions in the concentration range
10 −1 -10 −4 M. The useful pH range for this assembly is 6-10. The membrane electrode
shows good selectivity to sulphate ions and there is practically no interference of a large
number of anions and cations.

10.17.5 Ion chromatography
The application of this technique is discussed under multianion analysis in section 12.8.1.

10.18 Sulphide

10.18.1 Titration method
A sulphide ion selective electrode has been used [58] as an indicator electrode in the
potentiometric titration of sulphide in paper mill effluents with standard mercuric
chloride. In 0.1N sodium hydroxide medium, sulphide and polysulphide are titrated. As
the electrode responds to mercuric ions, thiosulphate and sulphite can also be titrated at
pH 7.0 to 7.5. The polysulphide plus sulphite can be converted to thiosulphate and can be
titrated. Sulphite can be decomposed with formaldehyde and, consequently, thiosulphate
can be titrated alone.
A standard UK method [59] describes an idiometric titration procedure for the
determination of sulphide.
In this method the sample is acidified and liberated hydrogen sulphide swept by a
stream of carbon dioxide into a solution of zinc acetate. Zinc sulphide formed is oxidised
by the addition of excess 0.00417M potassium iodate-iodine and unused iodine back
titrated with standard 0.0126M sodium thiosulphate. Up to 40mg L −1 sulphide can be
determined with a detection limit of 0.20mg L −1 . The standard deviation is 0.22mg L −1
sulphide at the 10.3mg L −1 sulphide level. Anions such as sulphite, thiosulphate and
dithionate can produce sulphur dioxide on acidification which if carried over into the zinc
acetate will react with iodine causing a positive bias in results. Formaldehyde forms an
addition compound with sulphur dioxide which is unreactive towards iodine under the
test conditions and its use in this procedure is effective for at least 1000mg L −1

sulphite

(as SO 3 ). Levels of sulphite below 50mg L −1 do not interfere.
Sulphides of silver, mercury and copper are insoluble under the test conditions so their
presence will not be detected. Caution is also needed in the case of samples containing
sulphides of cadmium, lead, nickel, cobalt, antimony and tin. Any volatile compounds
which react with iodine will interfere.

10.18.2 Spectrophotometric method
In this method [60] the sulphide in the sample is reacted with N,N diethyl-p-phenylene
diamine and potassium dichromate to form a blue colour with an absorption maximum at